Add basic support for typing_extensions.TypeVar (#14313)

This PR only adds the existing `TypeVar` support for the
`typing_extensions` variant.
I.e. it does not include support for `default` or `infer_variance`.

Fixes #14312

mypy/semanal.py

@@ -3847,7 +3847,7 @@ def process_typevar_declaration(self, s: AssignmentStmt) -> bool:
         Return True if this looks like a type variable declaration (but maybe
         with errors), otherwise return False.
         """
-        call = self.get_typevarlike_declaration(s, ("typing.TypeVar",))
+        call = self.get_typevarlike_declaration(s, ("typing.TypeVar", "typing_extensions.TypeVar"))
         if not call:
             return False
 

test-data/unit/check-generics.test

@@ -2663,3 +2663,36 @@ def foo(x: A) -> A:
                 # N: (Hint: Use "B" in function signature to bind "B" inside a function)
         return y.x
     return bar()[0]
+
+
+-- TypeVar imported from typing_extensions
+-- ---------------------------------------
+
+[case testTypeVarTypingExtensionsSimpleGeneric]
+from typing import Generic
+from typing_extensions import TypeVar
+
+T = TypeVar("T")
+
+class A(Generic[T]):
+    def __init__(self, value: T) -> None:
+        self.value = value
+
+a: A = A(8)
+b: A[str] = A("")
+
+reveal_type(A(1.23))  # N: Revealed type is "__main__.A[builtins.float]"
+
+[builtins fixtures/tuple.pyi]
+
+[case testTypeVarTypingExtensionsSimpleBound]
+from typing_extensions import TypeVar
+
+T= TypeVar("T")
+
+def func(var: T) -> T:
+    return var
+
+reveal_type(func(1))  # N: Revealed type is "builtins.int"
+
+[builtins fixtures/tuple.pyi]

test-data/unit/lib-stub/typing_extensions.pyi

@@ -1,10 +1,11 @@
-from typing import TypeVar, Any, Mapping, Iterator, NoReturn as NoReturn, Dict, Type
+import typing
+from typing import Any, Mapping, Iterator, NoReturn as NoReturn, Dict, Type
 from typing import TYPE_CHECKING as TYPE_CHECKING
 from typing import NewType as NewType, overload as overload
 
 import sys
 
-_T = TypeVar('_T')
+_T = typing.TypeVar('_T')
 
 class _SpecialForm:
     def __getitem__(self, typeargs: Any) -> Any:
@@ -25,6 +26,8 @@ Literal: _SpecialForm = ...
 
 Annotated: _SpecialForm = ...
 
+TypeVar: _SpecialForm
+
 ParamSpec: _SpecialForm
 Concatenate: _SpecialForm